Form birefringent cutoff polarizer and method

Abstract

The cladding and a portion of the core of a length of optical fiber are removed to form an interaction region. A form birefringent stack is positioned on the exposed core. The refractive index of the stack for polarizations parallel to the plane of the interaction region is approximately equal to the refractive index of the fiber core so that these polarizations radiate from the fiber. The refractive index of the stack for the polarization normal to the plane fo the interaction region is approximately equal to the refractive index of the cladding so that this polarization remains guided by the fiber. A photodetector forms an error signal from the polarization radiated from the fiber, and control circuitry processes the error signal to drive a polarization controller that adjusts the polarization of light input to the polarizer to minimize the error signal.

What is claimed is: 
     
       1. A polarizer for propagating an optical signal of a selected polarization in an optical fiber having a central core and a cladding surrounding the central core and for radiating optical signals of other polarizations from the optical fiber, comprising: an interaction region formed in a length of said optical fiber, said interaction region comprising a region of said optical fiber from which portions of the cladding and core have been removed; and   interaction means comprising alternating layers of at least a pair of isotropic materials for forming an asymmetrical refractive index distribution in said core at said interaction region for waves having a first polarization such that waves of said first polarization radiate from said optical fiber and for forming a symmetrical refractive index distribution in said interaction region for waves of a second polarization such that waves of said second polarization are guided in said fiber through said interaction region.   
     
     
       2. The polarizer of claim 1 wherein said length of optical fiber includes a convex portion and said interaction region comprises a flattened region on said convex portion. 
     
     
       3. The polarizer of claim 2 wherein said interaction region comprises: a planar core portion of said optical fiber having no cladding thereon; and   a planar cladding portion surrounding said planar core portion.   
     
     
       4. The polarizer of claim 3 wherein said interaction means comprises a first set of alternating layers of a pair of isotropic materials having first and second refractive indices arranged to comprise a form birefringent stack adjacent said planar core portion. 
     
     
       5. The polarizer of claim 4 wherein said stack of form birefringent material has a first refractive index that is essentially the same as the refractive index of said core and a second refractive index that is essentially the same as the refractive index of said cladding. 
     
     
       6. The polarizer of claim 4 wherein said form birefringent stack has a first refractive index that is essentially the same as the refractive index of said core for polarizations perpendicular to the interaction region and a second refractive index that is essentially the same as the refractive index of said cladding for polarizations parallel to said interaction region. 
     
     
       7. A polarizer of claim 2, further including a substrate having a curved groove therein, said optical fiber being mounted in said curved groove, said flattened region of said optical fiber being coplanar with a surface of said substrate. 
     
     
       8. The polarizer of claim 1 wherein said interaction means comprises alternating layers of two materials having different refractive indices. 
     
     
       9. The polarizer of claim 8 wherein said two materials comprise B 2  O 3  and GeO 2 . 
     
     
       10. The polarizer of claim 8 wherein said two materials comprise SiO 2  and GeO 2 . 
     
     
       11. The polarizer of claim 1 further including: means for forming an error signal in response to light radiated from said optical fiber; and   means responsive to the error signal for adjusting the polarization of light input to said interaction region.   
     
     
       12. The polarizer of claim 11 wherein said means for forming an error signal includes at least one photodetector for producing an electrical signal indicative of the intensity of light emitted from said optical fiber. 
     
     
       13. The polarizer of claim 11 wherein said means for forming an error signal includes: a first photodetector responsive to the intensity of light emitted from said optical fiber in a first direction; and   a second photodetector responsive to the intensity of light emitted from said optical fiber in a second direction.   
     
     
       14. The polarizer of claim 11 wherein said means for adjusting the polarization of light input to the interaction region includes means for controlling the refractive indices of a length of said optical fiber. 
     
     
       15. The polarizer of claim 14 wherein said means for controlling the refractive indices of a length of said optical fiber includes means for compressing said optical fiber to produce stress-induced birefringence. 
     
     
       16. The polarizer of claim 15 wherein said means for compressing said optical fiber includes a plurality of fiber squeezers. 
     
     
       17. A method for polarizing light propagating in an optical fiber having a central core and a cladding surrounding the core, comprising the steps of: forming an interaction region in a length of said optical fiber by removing a portion of the cladding and a portion of the core therefrom;   forming an asymmetrical refractive index distribution for optical waves having a first polarization such that said waves having said first polarization are radiated from said optical fiber at said interaction region;   forming a symmetrical refractive index distribution for optical waves having a second polarization such that said waves having said second polarization are guided in said optical fiber through said interaction region; and   placing a stack of alternating layers of isotropic material adjacent said interaction region to form said asymmetric and symmetric refractive index distributions.   
     
     
       18. The method of claim 17, further including the step of forming the interaction region at a flattened region on a convexly curved portion of said optical fiber. 
     
     
       19. The method of claim 18, wherein the step of forming said interaction region comprises the step of removing material from said optical fiber to form a planar core portion having no cladding thereon. 
     
     
       20. The method of claim 19, further including the step of placing an interaction material adjacent a part of said planar core portion. 
     
     
       21. The method of claim 20, further including the step of forming said interaction material of alternating layers of materials having different refractive indices. 
     
     
       22. The method of claim 21 wherein one of said layers comprises SiO 2  and the other layer comprises GeO 2 . 
     
     
       23. The method of claim 21 wherein one of said layers comprises B 2  O 3  and the other layer comprises GeO 2 . 
     
     
       24. The method of claim 18, further including the steps of: forming a curved groove in a substrate;   mounting said optical fiber in said curved groove; and   forming said flattened region of said optical fiber to be coplanar with a surface of said substrate.   
     
     
       25. The method of claim 17, further including the steps of: forming an error signal in response to light radiated from said optical fiber; and   adjusting the polarization of light input to said interaction region in response to the error signal.   
     
     
       26. The method of claim 25, further including the step of placing at least one photodetectoer in the optical path of light emitted from said optical fiber at said interaction region for producing an electrical signal indicative of the intensity of light emitted from said optical fiber. 
     
     
       27. The method of claim 25, further including the steps of: detecting the intensity of light emitted from said optical fiber in a first direction; and   detecting the intensity of light emitted from said optical fiber in a second direction.   
     
     
       28. The method of claim 25 wherein the step of adjusting the polarization of light input to the interaction region includes controlling the refractive indices of a length of said optical fiber. 
     
     
       29. The method of claim 28 wherein the step of controlling the refractive indices of a length of said optical fiber includes compressing said optical fiber to produce stress-induced birefringence. 
     
     
       30. The method of claim 29 wherein the step of compressing said optical fiber includes squeezing the optical fiber with a plurality of fiber squeezers. 
     
     
       31. The method of claim 30 including the steps of: aligning a first fiber squeezer and a second fiber squeezer to apply parallel stresses to said optical fiber;   placing a third fiber squeezer between the first and second fiber squeezers; and   aligning the third fiber squeezer to apply a stress to the optical fiber that is perpendicular to the optical fiber and at an angle of 45 degrees to the stresses from the first and second fiber squeezers.   
     
     
       32. A method for fabricating a polarizer for polarizing light propagating in an optical fiber having a central core and a cladding surrounding the core, comprising the steps of: removing the cladding and a portion of the core of said length of optical fiber to form an interaction region in said optical fiber; and   placing a birefringent body formed of layers of isotropic material adjacent said interaction region to provide an asymmetrical refractive index distribution for a first polarization and a symmetrical refractive index distribution for a second polarization.   
     
     
       33. A method for polarizing light propagating in an optical fiber having a central core and a cladding surrounding the core, the cladding having a cladding refractive index and the core having a core refractive index, comprising the steps of: removing a cladding portion and a core portion from a length of the optical fiber to form an interaction region;   replacing the core portion with a form birefringent stack having a pair of polarization-dependent refractive indices such that the core at the interaction region has an effective diameter less than the cutoff diameter required to guide waves of a first polarization in the optical fiber while having an effective diameter at least as large as the cutoff diameter required to guide waves of a second polarization.   
     
     
       34. The method of claim 33 including the step of forming the form birefringent stack to have a first refractive index substantially equal to the cladding refractive index for optical waves of the first polarization and a second refractive index substantially equal to the core refractive index for optical waves of a second polarization in the optical fiber such that waves having the first polarization are radiated from said optical fiber at said interaction region and optical waves having the second polarization are guided in said optical fiber through said interaction region. 
     
     
       35. Apparatus for polarizing light propagating in an optical fiber having a central core and a cladding surrounding the core, the cladding having a cladding refractive index and the core having a core refractive index, comprising: a length of the optical fiber from which a cladding portion and a core portion have been removed to form an interaction region;   a form birefringent stack placed adjacent the fiber core at the interaction region to replace the core portion, the form birefringent stack having a pair of polarization-dependent refractive indices such that the core at the interaction region has an effective diameter less than the cutoff diameter required to guide waves of a first polarization in the optical fiber while having an effective diameter at least as large as the cutoff diameter required to guide waves of a second polarization.   
     
     
       36. The apparatus of claim 35 wherein the form birefringent stack has a first refractive index substantially equal to the cladding refractive index for optical waves of the first polarization and a second refractive index substantially equal to the core refractive index for optical waves of a second polarization in the optical fiber such that waves having the first polarization are radiated from said optical fiber at said interaction region and optical waves having the second polarization are guided in said optical fiber through said interaction region. 
     
     
       37. A method for fabricating a polarizer for polarizing light propagating in an optical fiber having a central core and a cladding surrounding the core, the cladding having a cladding refractive index and the core having a core refractive index, comprising the steps of: removing a cladding portion and a core portion from a length of the optical fiber to form an interaction region;   replacing the core portion with a form birefringent stack having a pair of polarization-dependent refractive indices such that the core at the interaction region has an effective diameter less than the cutoff diameter required to guide waves of a first polarization in the optical fiber while having an effective diameter at least as the cutoff diameter required to guide waves of a second polarization.   
     
     
       38. The method of claim 37 including the step of forming the form birefringent stack to have a first refractive index substantially equal to the cladding refractive index for optical waves of the first polarization and a second refractive index substantially equal to the core refractive index for optical waves of a second polarization in the optical fiber such that waves having the first polarization are radiated from said optical fiber at said interaction region and optical waves having the second polarization are guided in said optical fiber through said interaction region.